1. Field of the Invention
The present invention relates to a two-layer copper polyimide substrate that is a material of electronic parts such as printed wiring boards, flexible printed circuits, TAB tapes, and COF tapes.
2. Description of the Prior Art
Polyimides have excellent heat resistance and are by no means inferior to other plastic materials with respect to mechanical, electrical and chemical characteristics, and therefore, are widely used as insulating substrate materials for electronic parts such as printed wiring boards (PWB), flexible printed circuits (FPC), tapes for tape automated bonding (TAB), and chip on films (COF). Such PWB, FPC, TAB or COF is obtained by processing a metal-coated substrate comprising a film, at least one surface of which is coated mainly with copper as a metallic conductive layer.
Metal-coated substrates include three-layer copper polyimide substrates comprising a polyimide film and a metal foil welded to each other via an adhesive and two-layer copper polyimide substrates comprising a polyimide film having a metal layer directly formed thereon. Further, two-layer copper polyimide substrates include casting substrates prepared by subjecting a commercially available copper foil to film formation and two-layer copper polyimide substrates prepared by plating comprising directly laminating a metal on a commercially available polyimide film by sputtering and electroplating or electroless plating or a combination of the both.
In recent years, especially, following miniaturization or thinning of portable electronic instruments, the foregoing TAB and COF are also required to realize miniaturization or thinning, i.e., high density, and their wiring pitches (wiring width/space width) are becoming narrow more and more. Therefore, two-layer copper polyimide substrates capable of making the thickness of a conductive layer (copper coat) thin and freely controlling it are watched.
However, though the two-layer copper polyimide substrates have an initial adhesion at a practically useful level, they are low in adhesions related to reliability, such as an adhesion in a heat resistant environment and an adhesion in a high-temperature and high-humidity environment, as compared with conventional three-layer copper polyimide substrates, and bring about uneasiness. Accordingly, the market requires the two-layer copper polyimide substrates to ensure a comparable adhesion (about 400 N/m) to three-layer copper polyimide substrates.